1. Field of the Invention
This invention relates to a charge pump circuit, specifically to a charge pump circuit provided with a negative electric potential generating charge pump circuit that generates a negative electric potential and a positive electric potential generating charge pump circuit that generates a positive electric potential.
2. Description of the Related Art
In general, a charge pump circuit is composed of multi-stages of pumping packets formed by connecting charge transfer MOS transistors in series so as to boost an input electric potential, and is widely used in circuits such as a power supply circuit for a drive circuit of a display device.
Some LSIs (large-scale integrated circuits) such as a drive circuit and the like require a positive electric potential relative to a ground electric potential VSS and a negative electric potential relative to the ground electric potential VSS. In this case, a negative electric potential generating charge pump circuit that generates the negative electric potential and a positive electric potential generating charge pump circuit that generates the positive electric potential are formed on a single P-type semiconductor substrate to form a charge pump circuit.
The negative electric potential generated by the negative electric potential generating charge pump circuit is applied to the P-type semiconductor substrate. The positive electric potential generating charge pump circuit is formed in an N-type well that is formed in a surface of the P-type semiconductor substrate. The positive electric potential is applied to the N-type well.
In the charge pump circuit described above, the positive electric potential generating charge pump circuit and the negative electric potential generating charge pump circuit have been controlled to start operation simultaneously or controlled so that the positive electric potential generating charge pump circuit starts operation first and then the negative electric potential generating charge pump circuit starts operation using the positive electric potential.
Detailed descriptions on the charge pump circuit are found in Japanese Patent Application Publication Nos. 2001-231249 and 2001-286125, for example.
However, the charge pump circuit described above has a problem of abnormal voltage boosting. The problem and its cause will be described referring to FIG. 14. An N-type well 11 is formed in a surface of a P-type semiconductor substrate 10. A charge transfer MOS transistor MP in the positive electric potential generating charge pump circuit is formed in the N-type well 11. Although there is a plurality of charge transfer MOS transistors in the charge pump circuit, FIG. 14 shows only the charge transfer MOS transistor MP in a final stage, which outputs an output electric potential HV of the positive electric potential generating charge pump circuit. The positive output electric potential HV of the positive electric potential generating charge pump circuit is applied to the N-type well 11 through an N+-type diffusion layer 12 formed in a surface of the N-type well 11.
An N-channel type MOS transistor MN is formed in the surface of the P-type semiconductor substrate 10 adjacent the N-type well 11. The N-channel type MOS transistor MN makes a clock driver that provides the negative electric potential generating charge pump circuit with a clock, for example, and its source, which is an N+-type diffusion layer 13, is provided with the ground electric potential VSS.
A P+-type diffusion layer 14 is formed in the surface of the P-type semiconductor substrate 10 adjacent the N-channel type MOS transistor MN. A negative output electric potential LV (a negative electric potential relative to the ground electric potential VSS) of the negative electric potential generating charge pump circuit is applied to the P-type semiconductor substrate 10 through the P+-type diffusion layer 14. The P-type semiconductor substrate 10 is biased around the ground electric potential VSS through the N+-type diffusion layer 13 when the negative electric potential generating charge pump circuit is not in operation.
When the positive electric potential generating charge pump circuit is put into operation, however, a substrate current flows into the P-type semiconductor substrate 10 by turning-on of a parasitic bipolar transistor 15, and an electric potential of the P-type semiconductor substrate 10 is raised to a more positive electric potential than the ground electric potential VSS. As a result, a forward current flows from the P-type semiconductor substrate 10 through a parasitic diode (P-N junction) made of the P-type semiconductor substrate 10 and the N+-type diffusion layer 13. The forward current makes a base current IB of a parasitic bipolar transistor 16, and the parasitic bipolar transistor 16 is turned on. This state is interpreted as that a thyristor composed of the parasitic transistors 15 and 16 is turned on.
Here, a drain diffusion layer of the charge transfer MOS transistor MP makes an emitter, the N-type well 11 makes a base and the P-type semiconductor substrate 10 makes a collector of the parasitic bipolar transistor 15. The N+-type diffusion layer 13 makes an emitter, the P-type semiconductor substrate 10 makes a base and the N-type well 11 makes a collector of the parasitic bipolar transistor 16.
When the thyristor is turned on, the positive electric potential generated by the positive electric potential generating charge pump circuit is reduced and the normal voltage boosting operation is not performed, because a stable current flows from an output end (output electric potential HV) of the positive electric potential generating charge pump circuit to the ground electric potential VSS through the P-type semiconductor substrate 10. Since the electric potential of the P-type semiconductor substrate 10 is raised, the output electric potential of the negative electric potential generating charge pump circuit is not reduced to a lower electric potential than the ground electric potential VSS and held in that state. Therefore, the negative electric potential generating charge pump circuit is also prevented from performing the normal voltage boosting operation.